In Angular services, it's common to use RxJS Subjects and Observables to manage state. However, these constructs do not naturally provide a way to retrieve their current value outside of subscription. This article explores how to access the current value in such scenarios.
Consider the following Angular service that utilizes a Subject to manage a boolean property isLoggedIn:
<code class="typescript">import {Subject} from 'rxjs/Subject'; import {Injectable} from 'angular2/core'; @Injectable() export class SessionStorage { private _isLoggedInSource = new Subject<boolean>(); isLoggedIn = this._isLoggedInSource.asObservable(); ... }</code>
In some cases, you may need to access the current value of isLoggedIn without subscribing to it. This is not possible with a regular Subject, as it only emits values to its subscribers and does not store its current state.
Solution: Using BehaviorSubject
To overcome this limitation, consider switching to BehaviorSubject, a specialized RxJS type that maintains an internal buffer to store the latest emitted value.
<code class="typescript">import {BehaviorSubject} from 'rxjs/BehaviorSubject'; @Injectable() export class SessionStorage { private _isLoggedInSource = new BehaviorSubject<boolean>(false); isLoggedIn = this._isLoggedInSource.asObservable(); ... }</code>
Compared to Subject, BehaviorSubject has two primary benefits:
Example with BehaviorSubject:
Using BehaviorSubject, you can access the current value of isLoggedIn even without subscribing:
<code class="typescript">const sessionStorage = new SessionStorage(); const isLoggedInCurrentValue = sessionStorage._isLoggedInSource.getValue(); console.log(isLoggedInCurrentValue); // True or False</code>
In summary, if you need to access the current value of an RxJS Subject or Observable, consider switching to BehaviorSubject, which provides both immediate emission for new subscribers and a getValue() method for direct retrieval.
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